The process by which white blood cells grow, divide and differentiate in the bone marrow is called hematopoiesis (Dexter and Spooncer, Ann. Rev. Cell. Biol., 3:423, 1987). Each of the blood cell types arises from pluripotent stem cells. There are generally three classes of blood cells produced in vivo: red blood cells (erythrocytes), platelets and white blood cells (leukocytes), the majority of the latter being involved in host immune defense. Proliferation and differentiation of hematopoietic precursor cells are regulated by a family of cytokines, including colony-stimulating factors (CSF's) such as G-CSF and interleukins (Arai et al., Ann. Rev. Biochem., 59:783-836, 1990). The principal biological effect of G-CSF in vivo is to stimulate the growth and development of certain white blood cells known as neutrophilic granulocytes or neutrophils (Welte et al., PNAS-USA 82:1526-1530, 1985, Souza et al., Science, 232:61-65, 1986). When released into the blood stream, neutrophilic granulocytes function to fight bacterial and other infection.
The amino acid sequence of human G-CSF (hG-CSF) was reported by Nagata et al. Nature 319:415-418, 1986. hG-CSF is a monomeric protein that dimerizes the G-CSF receptor by formation of a 2:2 complex of 2 G-CSF molecules and 2 receptors (Horan et al. (1996), Biochemistry 35(15): 4886-96). Aritomi et al. Nature 401:713-717, 1999 have described the X-ray structure of a complex between hG-CSF and the BN-BC domains of the G-CSF receptor. They identify the following hG-CSF residues as being part of the receptor binding interfaces: G4, P5, A6, S7, S8, L9, P10, Q11, S12, L15, K16, E19, Q20, L108, D109, D112, T115, T116, Q119, E122, E123, and L124. Expression of rhG-CSF in Escherichia coli, Saccharomyces cerevisiae and mammalian cells has been reported (Souza et al., Science 232:61-65, 1986, Nagata et al., Nature 319: 415-418, 1986, Robinson and Wittrup, Biotechnol. Prog. 11:171-177, 1985).
Leukopenia (a reduced level of white blood cells) and neutropenia (a reduced level of neutrophils) are disorders that result in an increased susceptibility to various types of infections. Neutropenia can be chronic, e.g. in patients infected with HIV, or acute, e.g. in cancer patients undergoing chemotherapy or radiation therapy. For patients with severe neutropenia, e.g. as a result of chemotherapy, even relatively minor infections can be serious and even life-threatening. Recombinant human G-CSF (rhG-CSF) is generally used for treating various forms of leukopenia/neutropenia. Thus, commercial preparations of rhG-CSF are available under the names filgrastim (Gran® and Neupogen®), lenograstim (Neutrogin® and Granocyte®) and nartograstim (Neu-up®). Gran® and Neupogen® are non-glycosylated and produced in recombinant E. coli cells. Neutrogin® and Granocyte® are glycosylated and produced in recombinant CHO cells and Neu-up® is non-glycosylated with five amino acids substituted at the N-terminal region of intact rhG-CSF produced in recombinant E. coli cells.
Various protein-engineered variants of hG-CSF have been reported (e.g. U.S. Pat. Nos. 5,581,476, 5,214,132, 5,362,853, 4,904,584 and Riedhaar-Olson et al. Biochemistry 35:9034-9041, 1996). Modification of hG-CSF and other polypeptides so as to introduce at least one additional carbohydrate chain as compared to the native polypeptide has been suggested (U.S. Pat. No. 5,218,092). It is stated that the amino acid sequence of the polypeptide may be modified by amino acid substitution, amino acid deletion or amino acid insertion so as to effect addition of an additional carbohydrate chain. In addition, polymer modifications of native hG-CSF, including attachment of PEG groups, have been reported (Satake-Ishikawa et al., Cell Structure and Function 17:157-160, 1992, U.S. Pat. Nos. 5,824,778, 5,824,784, WO 96/11953, WO 95/21629, WO 94/20069).
Bowen et al., Experimental Hematology 27 (1999), 425-432 disclose a study of the relationship between molecule mass and duration of activity of PEG-conjugated G-CSF mutein. An apparent inverse correlation was suggested between molecular weight of the PEG moieties conjugated to the protein and in vitro activity, whereas in vivo activities increased with increasing molecular weight. It is speculated that a lower affinity of the conjugates act to increase the half-life, because receptor-mediated endocytosis is an important mechanism regulating levels of hematopoietic growth factors.
The commercially available rhG-CSF has a short-term pharmacological effect and must therefore be administered once a day for the duration of the leukopenic state. A molecule with a longer circulation half-life would decrease the number of administrations necessary to alleviate the leukopenia and prevent consequent infections. Another, more significant problem with currently available rG-CSF products is that patients become neutropenic after chemotherapy even after administration of G-CSF. For these patients, it is important to be able to reduce the duration and degree of the neutropenic state as much as possible in order to minimize the risk of serious infections. A further problem is the occurrence of dose-dependent bone pain. Since bone pain is experienced by patients as a significant side effect of treatment with rG-CSF, it would be desirable to provide a rG-CSF product that does not cause bone pain, either by means of a product that inherently does not have this effect or that is effective in a sufficiently small dose that no bone pain is caused. Thus, there is clearly a need for improved recombinant G-CSF-like molecules.
With respect to the half-life, one way to increase the circulation half-life of a protein is to ensure that clearance of the protein, in particular via renal clearance and receptor-mediated clearance, is reduced. This may be achieved by conjugating the protein to a chemical moiety which is capable of increasing the apparent size, thereby reducing renal clearance and increasing the in vivo half-life. Furthermore, attachment of a chemical moiety to the protein may effectively block proteolytic enzymes from physical contact with the protein, thus preventing degradation by non-specific proteolysis. Polyethylene glycol (PEG) is one such chemical moiety that has been used in the preparation of therapeutic protein products. Recently, G-CSF molecule modified with a single, N-terminally linked 20 kDa PEG group (Neulasta™) was approved for sale in the United States. This PEGylated G-CSF molecule has been shown to have an increased half-life compared to non-PEGylated G-CSF and thus may be administered less frequently than current G-CSF products, but it does not reduce the duration of neutropenia significantly compared to non-PEGylated G-CSF. Thus, there is still substantial room for improvement of the known G-CSF molecules.
A need therefore still exists for providing novel molecules exhibiting G-CSF activity that are useful in the treatment of leukopenia/neutropenia, and which have are improved in terms of e.g. an increased half-life and in particular a reduction in the duration of neutropenia. The present invention relates to such molecules.